(a) Technical Field
The present disclosure relates to a fuel cell hybrid system. More particularly, it relates to a fuel cell hybrid system having a multi-stack structure, which maintains the voltage of a fuel cell at a level lower than that of an electricity storage means (supercapacitor) during regenerative braking so that the fuel cell preferably does not unnecessarily charge the electricity storage means, thereby increasing the amount of recovered energy and improving fuel efficiency.
(b) Background
A fuel cell is an electricity generation system that does not convert chemical energy of fuel into heat by combustion, but electrochemically converts the chemical energy directly into electrical energy in a fuel cell stack. A Fuel cell electricity generation system can be applied to the electric power supply of small-sized electrical and electronic devices, for example portable devices, as well as industrial and household appliances and vehicles.
One of the most widely used fuel cells for a vehicle is a proton exchange membrane fuel cell, or a polymer electrolyte membrane fuel cell (PEMFC), that includes a fuel cell stack comprising a membrane electrode assembly (MEA), a gas diffusion layer (GDL), a gasket, a sealing member, and a bipolar plate. Generally, the MEA includes a polymer electrolyte membrane through which hydrogen ions are transported. An electrode/catalyst layer, in which an electrochemical reaction takes place, is disposed on each of both sides of the polymer electrolyte membrane. The GDL functions to uniformly diffuse reactant gases and transmit generated electricity. The gasket functions to provide an appropriate airtightness to reactant gases and coolant. The sealing member functions to provide an appropriate bonding pressure. The bipolar plate separator functions to support the MEA and GDL, collect and transmit generated electricity, transmit reactant gases, transmit and remove reaction products, and transmit coolant to remove reaction heat, etc.
The fuel cell stack is composed of a plurality of unit cells, each of the unit cells including an anode, a cathode, and an electrolyte (electrolyte membrane). Hydrogen as fuel is supplied to the anode (“fuel electrode”, “hydrogen electrode, or “oxidation electrode”) and oxygen as oxidant is supplied to the cathode (“air electrode”, “oxygen electrode” or “reduction electrode”).
The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e−) by a catalyst disposed in the electrode/catalyst layer. The hydrogen ions are transmitted to the cathode through the electrolyte membrane, which is a cation exchange membrane, and the electrons are transmitted to the cathode through the GDL and the bipolar plate.
At the cathode, the hydrogen ions supplied through the (polymer) electrolyte membrane and the electrons transmitted through the bipolar plate react with the oxygen in the air supplied to the cathode to produce water. Migration of the hydrogen ions causes electrons to flow through an external conducting wire, which generates electricity and heat.
In general, fuel cell hybrid vehicles including large vehicles such as buses, as well as small vehicles, have a system equipped with an electricity storage means such as a high voltage battery or a supercapacitor as an auxiliary power source for providing the power required to drive a motor in addition to the fuel cell as a main power source.
At present, a fuel cell-supercapacitor hybrid vehicle which does not employ a power converter has been studied. A fuel cell-supercapacitor hybrid vehicle would preferably have, for example, high fuel efficiency (high regenerative braking, high efficiency of supercapacitor, and without the use of the power converter), an increase in durability of the fuel cell, high reliability control (automatic power assist and automatic regenerative braking function).
Preferably, in the hybrid vehicle in which the fuel cell and the supercapacitor are directly connected, the fuel cell continuously outputs power at a constant level during driving. If there is surplus power, the supercapacitor is charged with the surplus power, whereas, if there is insufficient power, the supercapacitor supplies the insufficient power to drive the vehicle.
The driving modes of the hybrid vehicle including the fuel cell as the main power source and the supercapacitor as the auxiliary power source includes an electric vehicle (EV) mode in which the motor is driven only by the power of the fuel cell, a hybrid electric vehicle (HEV) mode in which the motor is driven by the fuel cell and the supercapacitor at the same time, and a regenerative braking (RB) mode in which the supercapacitor is charged.
The above-described fuel cell-supercapacitor hybrid vehicle has the following problems during regenerative braking.
FIG. 1 is a schematic diagram showing an exemplary configuration of a conventional fuel cell-supercapacitor hybrid system. As shown in the figure, a fuel cell 10 as a main power source is connected in parallel to a supercapacitor 20 as an auxiliary power source to supply electrical energy to a motor 32, and the electrical energy generated by regenerative braking is suitably stored in the supercapacitor 20 through an inverter 31. At this time, the regenerative braking energy is not supplied to the fuel cell 10 by a blocking diode 50 for blocking a reverse current flowing to the fuel cell 10 but recovered to the supercapacitor 20.
The fuel cell-supercapacitor hybrid vehicle has limitations in the regenerative braking since the supercapacitor is automatically charged by the fuel cell. During braking of the vehicle, a considerable amount of regenerative braking energy generated in the driving motor is supplied and stored in the supercapacitor; however, since the fuel cell has no load at this time, the voltage of the fuel cell increases, which results in an increase in electrical energy of the supercapacitor.
The supercapacitor can store a greater amount of regenerative braking energy supplied from the driving motor, if the amount of stored electrical energy is smaller. Therefore, in order for the supercapacitor to store a greater amount of regenerative braking energy, the amount of electrical energy charged in the supercapacitor by the fuel cell should be suitably reduced during regenerative braking, and accordingly it is possible to prevent a decrease in the fuel efficiency.
Accordingly, where the electrical energy supplied from the fuel cell is suitably charged in the supercapacitor, the supercapacitor cannot store a considerable amount of regenerative braking energy, which is an important factor that decreases the fuel efficiency.
The surplus energy, suitably released when the kinetic energy of the vehicle by deceleration is not sufficiently recovered as electrical energy, is consumed as frictional heat in brake pads, which results in a suitable decrease in the durability of various parts of a brake system.
As described above, during the regenerative braking, the fuel cell should not charge the supercapacitor with electrical energy, and, accordingly, the voltage of the fuel cell should be lower than that of the supercapacitor. Since no electrical energy is drawn from the fuel cell during braking of the vehicle, the voltage of the fuel cell is gradually increased to reach an open circuit voltage (OCV) value; further, the electrical energy of the fuel cell is suitably supplied to the supercapacitor until the voltage of the supercapacitor increases. However, if the voltage of the fuel cell is higher than that of the supercapacitor, excessive energy (regenerative energy+fuel cell energy) is supplied to the supercapacitor, and thereby no more regenerative braking energy is supplied and stored in the supercapacitor.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.